The catalysts generally employed for the oligomerization of ethylene are the result of the combination of Group VIII metal compounds and metal alkyl reducing agents. Examples are disclosed in U.S. Pat. No. 4,032,590.
There have been disclosures of the use of soluble complexes of transition metal esters and trialkyl aluminums as catalysts for the dimerization and oligomerization of ethylene. See for example, U.S. Pat. No. 2,943,125 and Journal Of Polymer Science, Vol. XXXIV, pp. 139-151 (1959).
Such transition metal based dimerization catalysts have even been proposed for use with polymerization catalysts to result in the in-situ dimerization of ethylene so that ethylene copolymers could be made without the employment of a separate comonomer stream. See U.S. Pat. No. 4,133,944 and Journal Of Polymer Science, Vol. XXII, pp. 3027-3042 (1984). These liquid transition metal catalyst compositions have been found to have a rapid drop-off in activity. In addition they have been found to tend to poison polymerization catalysts, thus limiting their usefulness in the cojoint dimerization and polymerization of ethylene.